Oh I see.  I'm at a loss then as to why the build is failing.  A possible clue is the errors like:

profiling:/var/lib/buildbot/slaves/enable-optimizations-bot/3.x.gps-debian-profile-opt.nondebug/build/Objects/setobject.gcda:Merge mismatch for function 10

A Google search brings up this Stackoverflow page:

https://stackoverflow.com/questions/2590794/gcov-warning-merge-mismatch-for-summaries

To me, that implies that there are compiler outputs (.o, .a, etc) that were build from a previous version source code.  However, the first part of the log shows that the build does "make clean" so I don't see how that is possible.

Is there a way to run "git show 4e38d71a2b7 | git apply -R" on the built host and then try to re-build?